The FBI director is due to appear on Monday morning before a congressional committee which will ask him whether Donald Trump was wiretapped and whether there was collusion between the Trump campaign and Moscow.

James Comey’s appearance in the House of Representatives, alongside intelligence chiefs past and present, will be a climactic moment in the investigation of Russia’s role in last year’s US presidential election, which has dogged the early weeks of the Trump administration. But it is unclear how many of the key unanswered questions will be resolved.

One of the first questions Comey is likely to be asked is whether Trump or his campaign was subject to a wiretap, as the president has repeatedly claimed over the past fortnight, without providing evidence. Comey is reported to have told members of Congress in private that there is no grounds for that claim and ABC News predicted on Sunday he would say so officially in the first few minutes of his testimony.

If so, it would be a striking repudiation of a sitting president’s claims by his own FBI director, coming on the heels of a heated denial from the British government and its electronic intelligence agency, GCHQ, that it had spied on Trump on the Obama administration’s behalf. That was a claim made by a Fox News commentator, which had been read out at a White House briefing by spokesman, Sean Spicer.

“I hope that we can put an end to this wild goose chase, because what the president said was just patently false,” Adam Schiff, the ranking Democrat on the House intelligence committee, told the NBC’s Meet the Press. “It’s continuing to grow in terms of damage, and he needs to put an end to this.”

In a series of early-morning tweets on Monday, Trump claimed that the idea that he “colluded with Russia” was “FAKE NEWS and everyone knows it” and said that the former director of national intelligence James Clapper had said there was no evidence of such collusion. (Clapper has said he has seen no evidence but it “could have unfolded or become available in the time after I left the government”.)

Trump claimed that the Democrats had “made up and pushed the Russian story as an excuse for running a terrible campaign”.

“The real story that Congress, the FBI and all others should be looking into is the leaking of Classified information,” he added. “Must find leaker now!”

Comey will be joined at Monday’s committee hearing by Clapper, plus director of the national security agency Adm Michael Rogers, and John Brennan, the Obama administration’s CIA director.

The US intelligence community came to the conclusion in December that Vladimir Putin’s government had intervened in the election with the intention of skewing it in Trump’s favour. Comey and the intelligence chiefs will be questioned on Monday on the scope of that intervention and whether members of his campaign or any of his associates had colluded with Moscow.

On that explosive issue, there are less likely to be definitive answers. Counterintelligence investigations can continue for months or years without leading to a public conclusion or any arrests.

There is no question there were contacts between Trump aides and Russian officials, despite blanket denials of such contacts by the Trump camp. The administration’s first national security advisor, Michael Flynn, was forced to resign in mid-February over his communications with the Russian ambassador in Washington, Sergei Kislyak, and for failing to give an accurate account of them in public or to the vice-president, Mike Pence. The conversations were intercepted by US authorities and their contents leaked, demonstrating that they had discussed punitive measures imposed on Russia by the outgoing Obama administration, something Flynn had denied.

Flynn is not due to appear before the committee, but the Flynn affair is certain to be covered. Another of the witnesses appearing on Monday will be Sally Yates, former acting attorney general, who is reported to have warned the White House in late December that Flynn was vulnerable to blackmail because of his contacts with Kislyak.

Yates – who was fired for refusing to defend Trump’s travel ban – will be asked about those warnings, which if confirmed will raise questions over why Trump and his team ignored them until the story leaked to the press and whether Trump authorised Flynn’s contacts, something he has denied.

The intelligence committee hearings will also provide an arena for a partisan clash between Schiff, who will seek to focus the hearing on potential Trump campaign collusion with Russia, and the intelligence committee’s Republican chairman, Devin Nunes, who wants to concentrate on the string of leaks from the intelligence agencies about the Trump camp’s Moscow contacts.

“That’s the only crime we know has been committed right now,” Nunes said on Fox News Sunday.

Schiff said that the congressional investigation into hacking had so far turned up “circumstantial evidence of collusion” and direct evidence that the Trump campaign aides had sought to cover up contacts with Russians.

“There is certainly enough for us to conduct an investigation,” Schiff said. “The American people have a right to know and in order to defend ourselves, we need to know whether the circumstantial evidence of collusion and direct evidence of deception is indicative of more.”